Tracing shape memory effect and elastic bending in a conformationally flexible organic salt†

Molecular crystals which respond to multiple external stimuli are of great research interest as next-generation energy transducing smart materials. This article introduces a polymorphic organic salt, diphenyl phosphate anion and o-chloroaniline cation where a single crystalline form of this salt displays thermoresponsive (such as shape memory effect, bending, jumping, and splitting) and mechanoresponsive (elastic flexibility) behaviour. The single crystal of Form I shows a reversible phase transformation (below room temperature) that enables investigation of the significance of flexible torsions that result in conformational and rotational changes in the crystalline assembly. This cooperative transformation with almost negligible fatigue while undergoing a temperature-dependent shape memory effect also exhibits mechanical compliance at room temperature, which contrasts with the behavior exhibited by the other form. Intriguingly, the three-point bending test allows for determination of the elastic modulus at varying temperatures over which phase transformation occurs, rationalising the shape memory effect between the parent-daughter phase. Nanoindentation experiments are used to quantify and rationalize the mechanical response of these polymorphs, with the softer polymorph with lower resistance to elastic deformation showing mechanical compliance. These values corroborated well with the DFT-calculated elastic modulus values which predict the ranks of elasticity for the polymorphs. This study emphasizes the significance of multi-functional molecular crystals with improved understanding of hybrid properties.